Abstract. Confining a given number of magnets with vertically aligned moments on a non-magnetic surface creates a 2D magnet array, self-organized by repulsive forces in a hexagonal crystal-like network. Increasing areal density leads to a dramatic collapse of the structure where magnets finally stick together. We study the origin of this collapse and its critical density both experimentally, by either increasing the number of magnets or reducing the area, and theoretically, by using a dipole model. We suggest the collapse occurs when magnetic forces or torques created by irregularities overcome gravity. Transition from torque-driven to force-induced mechanism is observed when the critical density increases.

Single molecules e.g. in biological experiments can be detected by labeling with magnetic beads instead of the otherwise used dyes and by analyzing the influence of the beads on a GMRor TMR-multilayer. The development of suitable sensors has been studied experimentally as well as theoretically by computer simulation in order to optimize the sensor parameters. The local field method [l] with the dipole fields of the beads as additional contributions to the local field gt3k has been used in the simulations. The bead fields produce vortex like arrangements within the spin ensemble of a magnetic layer. There are 2 important fields: H, (in plane) and H, (out of plane).Paramagnetic heads show an induced dipol moment pg = moHo, therefore pg increases with H, and H, = 0 corresponds to the case without beads. In Figs. 1 and 2 we present theoretical graphs, which demonstrate some basic dependencies for a GMR-sensor (NiFe/Cu/NiFe).